The formula posted isn't the official calculation, just an estimate that someone came up with. Gas guzzler mpg's are calculated in a lab unlike regular mpg estimates, which results in over a 10% gain for city mpg's and over a 20% gain for highway mpg's, weighted 55% city 45% highway. Maybe someone in the industry or government could explain the whole thing better but I think I got the basics.

And just for fun a car could get 19/19 and still likely avoid the gas guzzler tax.

17/25 results in no gas guzzler tax.
For no penalty the total has to be above 22.5 for the following formula. (Do not ask me how they came up with this formula, there are reasons behind it all but it's from politicians and beaurocrats so don't look for to much logic)

Fi = 1/(.495/Fc+.351/Fh)+.15

Fi is our total that needs to be greater than 22.5
Fc is city mileage
Fh is highway mileage

17/25 results in 23.3+
With 17 city you can drop to 23 highway
With 16 city you have to get highway all the way to 26